1. Field of the Invention
This invention relates to an ophthalmic instrument provided with an eye fundus measuring portion for finding a position of the eye fundus of a patient's eye.
2. Description of the Prior Art
Heretofore, an ophthalmic instrument is known which is provided with an eye fundus measuring portion for finding a position of the eye fundus of a patient's eye.
To find the position of the eye fundus, this eye fundus measuring portion employs interference fringes which occur between low coherent light reflected from a reference mirror and low coherent light reflected from the eye fundus when positions of the reference mirror disposed in a reference optical path and the eye fundus coincide with each other. That is, the eye fundus measuring portion finds the position of the eye fundus such that alignment is first carried out, an optical system is then focused on the retina of the eye, the reference mirror is moved so as to discover a position at which interference fringes occur, and the position of the eye fundus is calculated from the interfering position of the reference mirror. Further, to measure an eye axis length (a length between the eye fundus and the cornea of the eye), a position of the cornea is found such that ring-shaped light is first projected onto the cornea, reflected light from the cornea is then received, and a corneal vertex position is calculated from the reflected light receiving position. Accordingly, the eye axis length is obtained as a difference between the positions of the eye fundus and the corneal vertex.
By the way, a plural number of light reflecting surfaces, such as the sclera, the choroid, and front and rear surfaces of the retina of the eye, line the eye fundus. In addition, the eye fundus is constantly moved by heartbeats of a patient, and thereby the eye axis length is changed.
Therefore, according to the heartbeats, the reflecting surface on which an optical system for projecting a laser beam onto the eye fundus is focused corresponds to any of the front and rear surfaces of the retina, the choroid, and the sclera of the eye. This results in a measurement error because the reflecting surface regarded as the eye fundus depends on operator's measuring timing.
For example, on the assumption that the retina is 0.2 mm thick, the difference between respective measured results obtained by light reflected from the front and rear surfaces of the retina is 0.2 mm. This difference cannot satisfy an accuracy 1/100 mm requisite for an instrument for measuring an eye axis length, and therefore the conventional measuring instrument could not accurately measure the eye axis length.